A typical disc drive storage system includes one or more magnetic discs which are mounted for co-rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disc. The transducer and the hydrodynamic bearing are sometimes collectively referred to as a data head or a product head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs. An electromechanical actuator operates within a negative feedback, closed-loop servo system to move the data head radially or linearly over the disc surface for track seek operations and holds the transducer directly above a desired track or cylinder on the disc surface for track following operations.
Often, the servo information used by the servo system is prerecorded on the disc surfaces during manufacture of the disc drive module using a process sometimes referred to as servo writing. Each disc drive module is mounted to a servo writer support assembly which precisely locates the disc surfaces relative to a reference or origin. The servo writer support assembly supports a position sensor such as laser light interferometer (for detecting the position of the actuator which locates the heads that perform servo track writing), and a push pin, driven by a servo writer voice coil, which positions the actuator itself. The position sensor is electrically inserted within the disc drive's negative feedback, closed-loop servo system for providing position information to the servo system while the servo data is being written to the disc surfaces. The servo writer support assembly may also support a clock writer transducer which writes a clock pattern onto the disc surface which is used for temporally spacing the servo data about the circumference of each track. A cleanroom is required during the servo writing process as the HDA needs to be unsealed to allow the clock head, push pin and laser to access the actuator and disk. This is to prevent particle contamination during servo writing. Servo track writer and cleanroom are very costly.
Another technique for writing servo information uses the disc drive itself to write the servo information in situ. In Situ recording means that the servo patterns are recorded on a fully assembled drive using the product head. This process is also referred to as selfservo writing. It removes the cleanroom requirement for conventional servo track writing, as the drive does not need to be open during the selfservo writing process. It also saves the conventional servo track writer cost. Calibration of product head geometry is important in both disc drives utilizing drive level servo implementations and in disc drives utilizing selfservo writing implementations. Measuring the width of the MR writer head or element helps in determining the widest head of the assembled drive, which in turn helps in determining the propagation step size in a checkerboard pattern of the type frequently used to implement selfservo systems. The read-write offset is an offset between a center line of the MR head and a center line of the writer head or element. In selfservo writing, measurement of the read-write offset helps in deciding where to write the next propagation burst. In conventional thin film head drives utilizing a single inductive element to read and write, the writer and reader are of the same width and there is no read-write offset. Thus, only the head width needs to be calibrated. In drives employing an MR read head, however, additional head parameters are critical, such as write head width, read head width and read-write offset.
Typically, calibration of MR head geometry at drive level is not difficult, because servo patterns are already available for position and displacement reference. However, in selfservo writing, there are no reference bursts or marks on the discs, which makes MR head calibration difficult. Moreover, due to quite large variations of the head specifications in drives, there exist different read-write offset cases at both the inner diameter (ID) and at the outer diameter (OD), which makes the calibration more complex. A method of calibrating MR head geometry using selfservo writing would be a significant improvement in the art.